(a) Field of the Invention
The present invention relates to an air-fuel ratio compensating device for internal combustion engines, and more particularly it relates to a compensation device incorporated in a flow measuring device arranged so that in order to accurately control the ratio of suction air to fuel so as to keep it at a constant value, the difference in the pressures existing on opposite sides of an air throttle valve placed in a suction pipe is kept at a constant value by a feedback control device, the amount of incoming air being determined by the opening area of said throttle valve.
Further, the invention relates to a fluid type or mechanical type compensation device which does not make it necessary to remodel said flow measuring device.
(B) Description of the Prior Art
In internal combustion engines, e.g., for automobiles, it is particularly important to engine efficiency and exhaust gas countermeasure to accurately control the weight ratio of suction air to fuel so as to keep it at a constant value. To this end, a high precision measuring device for measuring the amount of suction air is required. Generally, the conventional suction air flow measuring device for this purpose comprises a flow control valve interlocked to an accelerator pedal and placed in an air suction pipe, a flow detection valve interlocked to a fuel control mechanism and placed upstream of said flow control valve, said two valves defining an intermediate chamber, the pressure in said intermediate chamber being controlled by the opening and closing of the upstream valve so as to keep constant the difference in the pressures existing on opposite sides of said flow detection valve, so that the amount of suction air is proportional to the opening area of the flow detection valve, the amount of flow of air being thus determined by said opening area. The so-called area flowmeter system is known. The control of the pressure control valve in this system is performed by employing a pressure difference control servo-mechanism based on the feedback system wherein when the difference in the pressures existing on opposite sides of the valve is deviated from a predetermined value, the deviation is detected by the pressure difference setting diaphragm of the servo-mechanism, the detected value being then amplified by a fluid mechanism, the resulting amplified output being used to increase or decrease the degree of opening of the flow detection valve so as to bring said pressure difference to said predetermined value. Since incoming air varies in its specific weight with its temperature and pressure, a pressure difference compensating mechanism is incorporated so as to measure the amount of flow of air accurately.
While the air-fuel ratio provided by the above mechanism is appropriate during normal operation of the engine, often there arises the need of changing the air-fuel ratio so as to increase the same when the engine is at full throttle or the engine temperature is low. Thus, the above device is not appropriate.